You just never know how things would have been "if". Maradona could have stayed at Barca and ended up not as legendary, despite being the same person with same talents.
Also, comparing players of different era, back then, there were rules like no more than 3 foreigners in one team. Which meant large number of players are native and the gap in quality between the top teams and 3rd-4th teams were not like between Spurs and ManU. So saying "if Messi goes to Spurs, he won't win them title" is not fair, IMO.
As I said, back then, purchase of 3-4 top players would put you right in the mix of title contenders. Napoli had Careca, Alemao, bought Ferrara and Maradona etc. Conversely, I am sure that this Messi would have won that Napoli the title. No doubt about it. Speed of the game is much faster. Or to put it differently, I am sure Maradona wouldn't win a league title with Atletico Madrid in Spain.